Depression affects over 121 million people annually worldwide. Relatively low remission rates among depressive patients enforce the search for new therapeutic solutions and an urgent need to develop faster-acting antidepressants with a different mechanism of action occurs. The pathomechanism of depression postulated by the monoamine hypothesis is limited. The results of abnormalities in glutamate and γ-aminobutyric acid (GABA) systems in the brains of people with mood disorders allowed to develop new theories regarding pathophysiology of these disorders. Glutamatergic transmission is influenced by magnesium and ketamine through glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonistic effects. Magnesium and ketamine have a common mechanism of action in the treatment of depression: an increase in GluN2B (NMDAR subunit) expression is related to the administration of both of the agents, as well as inhibition of phosphorylation of eEF2 (eukaryotic elongation factor 2) in cell culture and increase of the expression of BDNF in the hippocampus. Combination of ketamine and magnesium in a normal magnesium level presents a superadditive effect in depression treatment. Analysed substances affect the GABAergic system and have anti-inflammatory effects, which is correlated with their antidepressant effect. The synergistic interaction between the pharmacodynamic activity of magnesium and ketamine may be of particular importance for patients with mood disorders. Further research is needed to determine the relationship between magnesium levels and ketamine treatment response mainly in the attempt to establish if the magnesium supplementation can change ketamine treatment response time or present superadditive effect.